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Chd|====================================================================
Chd|  SURF_AREA                     source/output/th/surf_area.F  
Chd|-- called by -----------
Chd|        HIST2                         source/output/th/hist2.F      
Chd|-- calls ---------------
Chd|====================================================================
      SUBROUTINE SURF_AREA(X   ,NN  ,SURF_NODES, AREA)
C-----------------------------------------------
C   I m p l i c i t   T y p e s
C-----------------------------------------------
#include      "implicit_f.inc"
C-----------------------------------------------
C   D u m m y   A r g u m e n t s
C-----------------------------------------------
      INTEGER NN,SURF_NODES(NN,4)
      my_real X(3,*), AREA
C-----------------------------------------------
C   L o c a l   V a r i a b l e s
C-----------------------------------------------
      INTEGER I,NOD1,NOD2,NOD3,NOD4
      my_real
     .   X13, Y13, Z13, X24, Y24, Z24,
     .   N1, N2, N3
C-----------------------------------------------
      AREA = ZERO
      DO I=1,NN
         NOD1=SURF_NODES(I,1)
         NOD2=SURF_NODES(I,2)
         NOD3=SURF_NODES(I,3)
         NOD4=SURF_NODES(I,4)
         X13= X(1,NOD3)-X(1,NOD1)
         Y13= X(2,NOD3)-X(2,NOD1)
         Z13= X(3,NOD3)-X(3,NOD1)
         X24= X(1,NOD4)-X(1,NOD2)
         Y24= X(2,NOD4)-X(2,NOD2)
         Z24= X(3,NOD4)-X(3,NOD2)
         N1 = Y13*Z24-Y24*Z13
         N2 = Z13*X24-Z24*X13
         N3 = X13*Y24-X24*Y13
         AREA = AREA + SQRT(N1*N1+N2*N2+N3*N3)
      ENDDO
      AREA = AREA * HALF
C
      RETURN
      END
